1. Technical Field
This invention relates to the art of making annular bearing assemblies with separable journalizing parts that permit reception of a shaft other than along an axial direction of the bearing; and, more particularly, it relates to the art of making split connecting rods for use in automotive internal combustion engines.
2. Description of the Prior Art
Automotive connecting rods usually have one end that forms part of an annular bearing assembly requiring separable cap and body portions to permit insertion of a complex configured crankshaft from a direction not along the axis of the bearing. A simple pin bearing assembly at the opposite end of the connecting rod usually attaches the rod to a piston; the simplicity of this attachment allows the pin to be received along the axis of the bearing.
The advent of more compact engines delivering higher horsepower at increasingly higher rpm's has placed increased stress on the connecting rod and its bearings. The bipartite rod should act as a unitary piece to transfer dynamic forces with better bearing life. To meet this challenge, the manufacture of automotive connecting rods has undergone evolutionary changes.
Connecting rods were originally made by separately casting or forging attachable cap and body portions. These parts were usually made of high carbon wrought steel and were separately machined at both joining faces and thrust faces; they were then separately with holes to accept fasteners. A first evolutionary step was to cast or forge the connecting rod as a single steel piece, followed by the drilling of holes to accept fasteners. The single piece was sawed to obtain cap and body portions which were separately rough-machined at the thrust and contacting surfaces; the two portions were then bolted together for finish-machining. The separate steps of machining and sawing were not only cumbersome and expensive, but they also did not ensure a perfectly matched cap and body under all operating forces. The bolts cannot ensure perfect doweling of the cap and body portions upon bolting together because of some diametrical clearance existing between the unthreaded bolt shank and the opening through which it extends. The bolts were torqued to apply compression forces that would prevent relative movement between the portions. Unfortunately, under some operating conditions, this inherent fastener clearance may permit slight microshifting of the cap and body portions which may affect bearing life.
As the next step in this evolutionary change, a single-piece connecting rod was split into its mating cap and body portions with an undulating interface in the hope of providing nonsliding surfaces where the cap and body portions are bolted together. If such surfaces were properly remated, the remate should prevent microshifting and assure more accurate operating alignment.
To split the single piece into two, it was initially struck on one side with a sharp blow. This met with little success because of the uncontrollability of the cracking plane and possible damage to the connecting rod. An early attempt at nonimpact splitting was accomplished by fracturing the big end of the connecting rods with a wedge-expandable mandrel placed in the crank bore opening (see U.S. Pat. No. 2,553,935). All finish machining was conducted on the one-piece connecting rod before fracturing. Even though the rod was made of a strong, nonbrittle, high carbon wrought steel, fracturing was carried out at room temperature. Brittleness across the cracking plane was achieved by cutting deep radial reductions at the crack plane--by sawing, milling, and drilling, or combination of all three--to significantly reduce the crackable section. Such connecting rods were intended for light duty applications such as small outboard marine engines and lawnmowers.
Another approach to splitting was disclosed in U.S. Pat. No. 3,751,080, which recognized the difficulty of fracturing strong high carbon steels at room temperature when they were formed in large sizes adequate for automotive engine applications. An electron beam was moved along a desired splitting plane in an undulating fashion to render a pair of rippled interfacing surfaces. Again, all machining was accomplished prior to the splitting. This technique may be undesirable not only because a high energy electron beam can have a deleterious effect upon material performance, but also because it is considered slower and more costly than previous techniques.
A recent attempt at splitting is disclosed in U.S. Pat. No. 4,569,109, which suggests that the rod can be composed of either cast iron, aluminum, or steel that is made brittle by freezing or heat treatment. Such materials can then be fractured by applying tension across a cracking plane while limiting relative movement of the cap and body portions to avoid bending or incomplete fracture (the material having sufficient ductility to provide this risk). Again, this method provides all finish-machining prior to cracking. Disadvantages peculiar to this technique are: (a) To avoid freezing or unnecessary heat treatment, cast iron or aluminum must be used that does not provide adequate tensile strength for a given size; thus, a more massive rod is necessary to achieve higher strengths, which is counterproductive both to fracturing at room temperature and to a better balanced rod. (b) Because all shaping or machining must be carried out prior to cracking, the technique suffers from association with wrought materials. (c) Separate machining must be provided to make the rod sensitive to cracking. (d) Marginally ductile materials cannot always be cleanly cracked. (e) Crack-initiating notches in the crank bore wall provide inadequate support for insertion of a bearing member.
There are also certain disadvantages common to all of the prior art splitting techniques: (a) the bolts, when assembled into both the cap and body portions of a split rod, are retained loosely in place until final assembly, subject to being unintentionally unscrewed and misplaced and thereby permitting mix-up of mating parts; (b) the clearance between each bolt shank and bolt opening is not controlled sufficiently to provide an adequate guide to remating the cap and body portions at identically their exact separation location; (c) some slight distortion in the roundness of the bore opening in the cap and body portions may accompany room temperature splitting by tension and is not compensated during reassembly thereby detracting from the accuracy of the final assembly; and (d) the need to machine locking notches in the internal surface of the bore opening wall while the cap and body are separated.